Ionizing radiation produces many molecular lesions in irradiated DNA. It is moot point whether any particular molecular lesion is irreparable and hence uniquely threatening to a cell. However, the lesions represent a potentially rich resource for studies of cell repair kinetics and, in selected cases, for studies of intracellular radiation chemistry. The availability of analyses for specific molecular lesions would provide an exciting new dimension to the study of intracellular radiation chemistry which is dominated at present by DNA strand break analysis. Without detracting from the value of strand break studies, it is also possible to state that the strand break process is chemically complex. As a result it is difficult to discern initiating radical events in the final, and often alkali-induced, strand break lesion. Our long range goal is to develop immunochemical probes based on specific molecular lesions which can inform us about the details of initial radical events in irradiated DNA- particularly as they relate to the radiation chemical model of radiation damage in cells. We hope, thereby to widen one of the scientific bases on which rational approaches to improved radiotherapy rest. Our immediate goals are to (1) use immunochemical probes for 8,5'- cyclodeoxyadenosine, 6,5' cyclothymidine and alpha-deoxyadenosine to test the radical model of radiation damage as it relates to radioprotection in isolated DNA and (2) develop the immunochemical technology to the point of application to studies of DNA in irradiated cells.